We’re seven episodes into the second season of “The Knick,” and the odd-couple relationship between rough-hewn ambulance driver Tom Cleary (Chris Sullivan) — he of the thick Irish brogue — and fallen nun Harriet (Cara Seymour) continues to reveal its surprises.

“There’s something very realistic about their relationship,” says Sullivan (sans the brogue — he was born and raised in California). “The way they came to know each other is a very unique bit of storytelling. Cleary would say, (Sullivan slips back briefly into his brogue), ‘As far as I can tell she’s done right by me so I’ll continue to do right by her.’”

Viewers first met the characters last season on “The Knick,” which airs on Cinemax and recounts life at a New York City hospital circa 1900. Cleary — the Ralph Kramden of his day, always looking to make a buck — earned extra dough by taking pregnant young women to Sister Harriet for secret (safer) abortions. She was eventually busted, de-frocked and sent to prison; Cleary was the only person who supported her, both emotionally and financially — an unpredictable turn, revealing a depth to his character no one saw coming. Harriet has now moved in with Cleary (no funny business) to partner with him in providing condoms (a rarity) to hookers and johns.

“I think it’s interesting because the audience desperately wants them to be together but also absolutely wouldn’t want them to be together,” says Sullivan. “It would be a real easy way to spark some salacious storytelling, but [series creators] Michael Begler and Jack Amiel have been holding off. I think it’s one of the more realized ‘love stories’ as far as whatever their definition of love is.”

Sullivan checked into “The Knick” after a career in theater (he headlined the national tour of “Defending the Caveman”) and steady voiceover work. (Fun fact: he’s the voice of the lovable Geico camel — “Hump day! Yeah!” — one of the best commercials in recent memory.)

He says he already had the thick beard when he auditioned for the role of Cleary. “It wasn’t a requirement,” he says. “Originally they envisioned [Cleary] as slimmer, a wiry guy with a big brush-broom mustache. The beard won their hearts. I joke that the beard does 40 percent of the job.”

So what’s in store for Cleary and Harriet as we head into the show’s final three episodes of the season?

“I think that no matter what your hopes are for the couple, you won’t be disappointed by the way Jack and Michael have handled their evolution,” Sullivan says. “Cleary is a highly principled person who often does bad things for good reasons.